I'd rather ride 100 miles in 60 degree weather, but if 100 degrees is what you got, goodonya for getting out there. In any case, riding in those conditions beats the stuffing out of running in 90 degree weather.

Yes, you experience adaptation. If you hydrate and recover appropriately, you get both the training adapatation, as well as adapting/adjusting/whatever to the heat so it will be easier next time. We do rides like that every weekend here in Phoenix, but likely with a drier heat than you experienced.

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"If you're riding less than 18 MPH up a 2% grade please tell people Coggan is coaching you."

That which doesn't kill you could still be a tactical mistake. In 100 degree weather I'd personally opt for a terrace and and a pitcher of sangria so I could watch other cyclists going by (preferably with a really high average speed). Evenings are cooler and better suited for 60km adventures.